Although many life-threatening drug reactions are reported each year, very little is known about their etiologies. We believe that many of these toxicities might be caused by the covalent alterations of specific macromolecules by reactive metabolites of drugs. These modifications might lead directly to toxicity by changing the physiological function of macromolecules or indirectly by making the macromolecules immunogenic, leading to immune-mediated toxicities. This year it was found that the trifluoroacetylated liver neoantigens associated with halothane hepatitis could be expressed in primary cultures of mouse or rat hepatocytes. This permitted the discovery that the level of expression of the neoantigens could be increased several fold by cellular stress. An increased level of expression of the neoantigens might influence their immunogenicity and the course of hepatic injury produced by halothane. It was concluded that analogues of halothane, HCFC-124 and HCFC-125, which have been developed as replacements for the ozone-depleting freons, might be safer for industrial use than the replacement, HCFC-123. This idea was based upon the finding that HCFC-124 and HCFC-125 produced approximately 10-50 fold lower levels of trifluoroacetylated liver neoantigens in rats than that found with HCFC- 123. We have continued to study the targets of reactive metabolites of nonsteroidal anti-inflammatory agents, one of the most widely used classes of drugs. Although relatively safe, idiosyncratic life-threatening hepatitis, nephritis, hemolytic anemia, or agranulocytosis is produced by some of these agents. With the use of an antibody raised against the prototype nonsteroidal anti-inflammatory agent diclofenac, it was found that a major liver target of diclofenac was a protein fraction of approximately 110 kDa, which was localized to the bile canalicular domain of the plasma membrane of hepatocytes. This finding may help explain in the future how diclofenac and other nonsteroidal anti-inflammatory agents cause life-threatening idiosyncratic hepatotoxicity.